Photochemistry of a dual-bacteriorhodopsin system in Haloarcula marismortui: HmbRI and HmbRII.

Recently, a dual-bacteriorhodopsin system, containing HmbRI and HmbRII, has been found in Haloarcula marismortui (Mol. Microbiol. 2013, 88, 551-561), and the light-driven proton pump activities were intrinsically different in a wide pH range. Compared with bacteriorhodopsin in H. salinarum (HsbR), the identical steady-state absorption contours of HsbR and HmbRs in the visible range indicated similarities in the retinal pocket. In addition, other reactive residues, including the proton relay channel, proton release group, and proton collecting funnel at the cytoplasm, were mostly conserved. We employed transient absorption spectroscopy and global analysis to characterize the photocycle intermediates and kinetics of HmbRI and HmbRII in the pH range of 4-8. The features of the time-resolved difference spectra of HmbRI indicated that the photocycle of HmbRI mainly followed the conventional pathway, including intermediates M, N, and O. A minute bypassed pathway from intermediate M needed to be included to better match the experimental data. The corresponding intermediate M' is attributed to the all-trans deprotonated Schiff base retinal, indicating the occurrence of retinal reisomerization prior to the reprotonation of the deprotonated Schiff base following the decay of intermediate M. Regarding HmbRII, its photocycle only followed the intermediates M and N, without intermediate O. The plausible molecular mechanisms, including the effects of the lengths of the loops and the distribution of the charged residues in the bacterio-opsin interior, were proposed to explain the differences in the photocycles. The pH-dependent photocycles were also investigated, and the results supported our proposed mechanism. Unravelling the photocycles of the HmbRs in the Haloarcula marismortui provided evidence that not only expanding the functional pH ranges but also the turnover kinetics are the strategies of the dual-bR system in the evolution of microbes in extreme environments.